The present invention relates to a device for measuring select physical properties of a moving sheet, and more particularly, to a single sided reflectance sensor which has an optical arrangement which permits accurate measurements of those properties across the width of the sheet, including near the edges of the sheet, and is substantially insensitive to sheet flutter.
One of the physical properties used in determining the quality of paper is the surface luster or gloss of paper. Various grades of paper having different surface gloss are produced to suit various applications. Another physical property is the amount of coating applied to a sheet of paper. Still another property is the moisture content of paper. During paper production, it is desirable to periodically or continuously measure one or more of these types of physical properties to ensure that the paper has the desired qualities.
Typically, such physical properties of paper are measured and controlled during paper production before the finished paper, which is manufactured in a continuous sheet, is packaged in the form of rolls. The rolls of paper, which are typically 25 feet or more in width, are then shipped to paper product manufacturers who may further process the paper in accordance with the intended use.
Certain devices for determining these physical properties include optical systems which direct a beam of electromagnetic radiation (e.g., visible light, ultraviolet light and/or infrared light) at a paper sheet and then measure the intensity of a beam at certain wavelengths of the electromagnetic spectrum reflected from the paper surface. Typically, the value of a select physical property of the paper is determined by comparing the intensity of one or more wavelengths of the reflected beam with the intensity of the same one or more wavelengths in the incident beam and/or the beam reflected from a standardization member having a known physical property. For example, in the case of measuring the gloss of a paper sheet, the intensity of the beam reflected from the paper is compared to the intensity of the beam reflected from a glass standardization tile having a polished surface with a known gloss. More specifically, in measuring the reflectance of the paper, the electromagnetic radiation beam is projected onto the paper surface, and a reflectance sensing means, such as a photocell or photoresistor, located on the same side of the paper and which is responsive to the intensity of certain electromagnetic radiation, measures the intensity of the electromagnetic radiation reflected from the paper surface. The sensor measures the reflectance of the standardization member in the same manner by substituting the standardization member for the paper surface. The reflectance of the paper surface is then referenced to the reflectance of the standardization member, thereby providing a measure of the gloss of the paper.
In practice, the reflectance measurement of the paper is typically made at approximately six inch intervals or "slices" as the sensor scans back and forth across the width of the paper. This spacing between measurements is usually sufficient to ensure uniformity of the select physical property in the cross-direction of relatively large paper rolls. For this purpose the electromagnetic radiation source may project a circular shaped beam of approximately 1.5-2.0 inches in diameter onto the paper surface.
Single sided reflectance sensors must meet certain conditions in order to obtain accurate measurements. The first condition is that the entire incident beam must be projected onto the paper surface; otherwise, the intensity of the reflected beam will not accurately indicate the value of the select physical property. For example, if any portion of the incident beam is projected beyond the edge of the paper, the detected intensity of the reflected beam will be reduced without any actual change in the magnitude of the select physical property. If the precise location of the paper edge is unknown, the source of the change in intensity will be unknown. Thus, measurements near the edge of the paper become inaccurate and unreliable.
This condition limits the ability to accurately measure the select physical properties near the edge of the paper, particularly where there is sheet movement in the cross-direction. At best, the center of a circular shaped beam cannot be located closer than half the diameter from the paper edge. Where the sensor is being used to measure and control the amount of coating applied to paper, for example, the unmeasurable portion of the coated paper near the edge must be trimmed away to avoid reel building problems.
The second condition is that the entire beam must be projected within the slice width to be controlled; otherwise, the intensity of the reflected radiation is not representative of the select physical property of the slice width. Thus, "tighter" cross-directional control, that is smaller than the cross-directional width of a standard beam, is not obtainable through use of a standard single sided reflectance sensor. This greatly limits the utility of standard single sided reflectance sensors in applications for plastic films and smaller photographic paper rolls, which are typically 2-5 feet in width, where tighter cross-directional control is either desirable or necessary.
Certain single sided reflectance sensors also require that, for accurate measurements, the paper remain a fixed distance from the sensor. The distance from the sensor to the plane of the sheet is called the "passline". Thus, any movement by the paper perpendicular to the passline must be minimized, that is, the paper must remain steady and not flutter. The problem of sheet flutter arises during paper manufacturing when the select physical property is monitored at a location where the paper is to some extent unsupported. Typically, the single sided reflectance sensor is housed in a protective enclosure located on one side of the moving paper. The rapidly moving paper creates a rapidly moving flow of air immediately adjacent and parallel to the surface of the sheet. Upon passing by the enclosure, the airflow is distorted generating uneven air pressure on one side of the sheet, thereby producing sheet flutter. Since the intensity of the detected portion of the reflected beam varies in some manner as the distance between the sheet and the sensor changes, sheet flutter produces inaccurate measurements in single sided reflectance sensors. Minimizing sheet flutter has therefore been required to maintain an acceptable degree of accuracy in measuring select physical properties of paper.